Catalysts used in the oxychlorination process are disclosed in U.S. Pat. Nos. 3,624,170, 4,446,249, 4,740,642, and 5,011,808 and European Patent Publication no. 0255156. An example of the conditions required in catalytic oxychlorination is described in U.S. Pat. No. 3,488,398 to Harpring et al.
U.S. Pat. No. 3,264,170 discloses an oxychlorination catalyst containing copper chloride, sodium chloride and magnesium chloride. EP-0255156 discloses an oxychlorination catalyst consisting of copper chloride, magnesium chloride, sodium and/or lithium chloride. U.S. Pat. No. 4,740,642 discloses an oxychlorination catalyst comprising copper, an alkali metal salt and a rare earth metal salt.
U.S. Pat. No. 5,011,808 discloses a non-sticking oxychlorination catalyst having improved EDC selectivity, consisting essentially of copper chloride, magnesium chloride and potassium chloride. The specified metal composition in weight percents are:
Cu: 3 to 9%, preferably 4 to 8% PA1 Mg: 0.2 to 3%, preferably 0.2 to 1.5% PA1 K: 0.2 to 3%, preferably 0.5 to 2%.
U.S. Pat. No. 5,011,808 teaches the above specified metal content for use under a given set of conditions, or under any conditions. Comparison between the inventive combination and controls was shown under fixed conditions, i.e. the same conditions were used for each catalyst and synergistic effects were seen in the improvement in ethylene selectivity to carbon oxides and EDC selectivity over those catalysts with either Mg or K alone with copper.
Comparisons of oxychlorination catalysts at standardized conditions provide a convenient way to illustrate differences in catalyst performance, but conclusions are limited to conditions near those controlled conditions. Such results may not reflect the highest potential performance of a catalyst which may require different. conditions for different catalysts. A truer test of performance for a particular catalyst is under conditions which are optimum for that particular catalyst. Many of the key parameters in the oxychlorination of ethylene to produce EDC are inherently coupled. Arriving at conditions for maximizing one performance parameter often comes at the expense of another. For example, conversion of ethylene can be controlled to 100% and it is well understood that increasing the reaction temperature will achieve this. However under the highest ethylene conversion conditions, the purity of the EDC product is reduced as more undesirable by-products are produced. Two key performance parameters in the oxychlorination process are EDC selectivity (selectivity of ethylene to form EDC) and ethylene efficiency and are expressed as follows: ##EQU1##
Also, the conversion of HCl must be maintained high in the best balance because of the problems attendant in handling and neutralizing any remaining HCl in the effluent. It is important, therefore, to find that set of conditions for a given catalyst that gives the best overall performance of that catalyst.
The realities of large scale production processes play a role in restricting the desired operating conditions. For example, in the air-based oxychlorination process, the ethylene conversion must be maintained at high levels to avoid excessive losses of ethylene. This can be accomplished by increasing the temperature of operation but this leads to reduced crude product purity and possibly reduced HCl conversion if the operating conditions are in a region beyond the maximum HCl conversion, which is likely. Specifically, the best balance of reaction conditions found for an oxychlorination catalyst are where the selectivity of ethylene conversion to EDC (EDC selectivity) is highest, breakthrough of unconverted feed lowest, and the percentage of by-product formation is lowest.
It has been found that as reaction bed temperatures increase, ethylene conversion increases to a maximum of 100% but a point of maximum HCl conversion is reached, beyond which HCl conversion decreases. Optimum conditions, unique to any oxychlorination catalyst are found at a point where ethylene conversion is at or above 99% and at a point where HCl conversion has not dropped severely. Near these conditions, the ethylene selectivity to EDC and HCl selectivity to EDC between different catalysts can be compared to arrive at conclusions as to which catalyst gives the better overall performance, compared to the other catalysts.
Another important commercial consideration pertains to the quality of fluidization. Even after an oxychlorination catalyst can be identified as having higher potential. performance than another, whether the improvement in the use of this catalyst in a commercial scale process can be realized depends on whether there is increased risk of that catalyst exhibiting stickiness in the selected operating range. If stickiness occurs, one can not safely operate in that range of conditions.
The inventors undertook a study of an oxychlorination reaction using the catalyst bed containing fluidized particles of a catalyst composition taught in U.S. Pat. No. 5,011,808. A catalyst, consisting of alumina on which was deposited 5% Cu, 0.5% K and 1.1% Mg, was employed in a series of experiments utilizing a fluid bed oxychlorination reactor. The optimum conditions were determined at a feed ratio of HCl to ethylene of 1.979, a bed temperature of from 220.degree. C. to 230.degree. C. and contact time of 25-26 seconds. Table C1 illustrates three of the experiments nearest the optimum. It can be seen that above 225.degree. C., all of the parameters evidence a decline in performance. At 225.degree. C., ethylene efficiency has peaked and HCl conversion is on a downward trend. The point nearest the optimum performance for this catalyst is at about a 225.degree. C. bed temperature.
U.S. Pat. No. 5,011,808 specifies a minimum magnesium content of 0.2% on a metal weight basis. A catalyst containing 5% CU, 0.5%k and 0.2% Mg was tested under a series of continuous oxychlorination reactions. Those conditions nearest the optimum performance are listed in Table C2. The key performance parameters measured, HCl conversion, ethylene efficiency, crude purity and percent 1,1,2-trichloroethane (by-product) are shown.
TABLE C1 ______________________________________ Temp HCl Ethylene Crude By-Product .degree.C. Conversion Efficiency Purity 1,1,2 ______________________________________ 220 99.18% 96.58% 99.59% 0.32% 225 98.27 97.48 99.44 0.45 230 98.19 97.20 99.23 0.62 ______________________________________
TABLE C2 ______________________________________ Temp HCl Ethylene Crude %% By-Product .degree.C. Conversion Efficiency Purity 1,1,2 ______________________________________ 210 98.8% 96.74% 99.67% 0.23% 215 98.75 96.8 99.47 0.33 220 98.61 96.3 99.34 0.43 ______________________________________
It is evident from tables C1 and C2 that as the magnesium content was reduced from 1% to 0.2% on a metal weight basis, the maximum obtainable ethylene efficiency dropped from 97.48% to 96.8%. One would suspect that at this reduced level of magnesium, that the synergistic interaction with potassium and magnesium was abating in view of U.S. Pat. No. 5,011,808. Further study of optimized performance of other catalysts at lower magnesium levels lead to an unexpected observance, which on a commercial scale will provide a significant improvement in the oxychlorination process. A commercial scale increase in ethylene efficiency of 0.5% can translate into a savings on the order of $200-500,000 annually.